Structure and bi-magnetism of nanocomposites and nanoalloys synthesized by reduction of (Co, Ni) Fe2O4 nano-ferrites.

Abstract

Structural and magnetic properties of CoFe2O4, NiFe2O4 nano-ferrites, CoFe2O4 /CoFe2, NiFe2O4/NiFe bi-magnetic nanocomposites and CoFe2, NiFe alloys were studied. The nano-ferrites were synthesised using the glycol-thermal method at 200 C for 6 hours. The nanocomposites and alloys were produced by the reduction of the ferrites using di erent amounts of activated charcoal. The reduction reaction was performed at 900 C for 3 hours in high purity owing argon atmosphere. Complete reduction yields the alloy phases while partial reduction produces the nanocomposites. The samples were studied using X-ray di raction (XRD), high resolution transmission electron microscopy (HRTEM), high resolution scanning electron microscopy (HRSEM), 57Fe M ossbauer spectroscopy, LakeShore vibrating sample magnetometer and a mini cryogen free system. XRD measurements of the nano-ferrites showed a single phase spinel structure with average crystallite sizes of about 10 nm. the CoFe2 alloy was obtained at an activated charcoal to ferrite molar ratio nC = 6 while the NiFe alloy was obtained at nC = 5. XRD results of the reduced samples of NiFe2O4 showed the coexistence of bcc -Fe and fcc -Fe lattice structures for NiFe alloy. The HRTEM and HRSEM measurements of all the reduced samples showed clear di erences between the morphology of the parent nano-ferrites and the reduced samples. 57Fe M ossbauer spectra measurements of the reduced samples showed the transformation of the spinel structure to the alloy phase. The presence of mixed phases in the nanocomposites was revealed. A small amount of mixed phase in CoFe2 and NiFe alloys which was not detected by XRD was revealed by M ossbauer measurement for nC = 6 and nC = 5 respectively. The M ossbauer ts showed that Fe3+ was reduced to Fe2+ with lower magnetic hyper ne elds observed in the reduced samples. High eld (50 kOe) magnetization measurements at room temperature for CoFe2O4/CoFe2 nanocomposite show signi cant enhancement of saturation magnetization from 63 emu/g of the parent nano-ferrite to 221 emu/g at nC = 5. In the NiFe2O4/NiFe nanocomposite the in magnetization increased from 57 emu/g to 141 emu/g at nC = 6. Low temperature measurements performed on the nanocomposites exhibited higher magnetizations. The coercivity of the fully reduced samples of the nano-ferrites were observed to be less dependent on temperature.